This is really too broad to answer. What kind of external drive are we talking about here? USB, Firewire, eSATA...? How about internal drives? IDE, SATA, 5200/7200/10000RPM. Lots of variables can affect their longevity. How often is the drive shut off? How often is it accessed? What's the case environment? Ambient temps and such...
I guess for the most part, external drives do tend to get warmer due to more confined space and are powered on and off rather frequently. However, I've seen some enclosures with some pretty darn good cooling, and there are some circumstances where the drive remains spinning even while the computer is off...
So, to answer your question... Internal drives are probably still the best option. However, realize there is no difference between an internal or external drive. An external drive is simply an internal drive in a case.
